Liquid metal ion source

ABSTRACT

A liquid metal ion source includes a cylindrical rod made of an electrically conductive refractory material, ending in a conical pointed end. The cylindrical rod passes through a reservoir of a liquid supply metal. The length of the rod inserted into the reservoir is in electrical contact with the liquid metal in the reservoir, and the reservoir is in contact with a conductive filament. The cylindrical rod, the reservoir and the conductive filament are electrically connected in series.

DESCRIPTION

1. Technical Field

This invention relates to the field of liquid metal ion sources, inwhich ions are produced from a supply metal which covers a point made ofa refractory metal.

By applying an intense electrical field, under vacuum, between thispoint and an extraction electrode, there is emission of ions accordingto a field evaporation mechanism. This emission is localised at the apexof the point. The size of the emissive area is of the order of a few nm²for an emission current of about 2 μA.

These sources are used in focused ion beam machines which play anincreasingly important part in the techniques of microelectronicfabrication.

At present, these machines almost exclusively use ion sources of liquidgallium metal. The use of lighter ions is interesting since it allowsthe size of ionic probes to be reduced. This allows the resolution ofexisting machines to be increased, by simply changing the source.Furthermore, for implantation applications, a light element isinteresting since it penetrates more deeply into the material than aheavy element, at equal energy.

2. State of the Art

The article by Bell et al. that appeared in the "Journal of AppliedPhysics", Vol. 53, No. 7, July 1982, pp. 4602 to 4605 described a sourceof aluminium ions made up of a graphite point fixed onto a tungstenheating filament. The graphite point is surface treated by deposition ofa film of titanium. In effect, the graphite is relatively resistant toattack by liquid aluminium, but it is difficult to wet it with such aliquid. The use of a film of titanium allows this problem to beresolved. The source has the shape illustrated in FIG. 1, wherereference number 2 designates the graphite point, which has, at its basea cylindrical part. A heating filament 4, made of tungsten, passesthrough this cylindrical part. Heating of the point 2 is obtained by theheat released by a Joule effect in the filament.

This type of source has the advantages, on the one hand of simplicity,and on the other hand, of compactness since the graphite point has adiameter of about 0.8 nm, for a length of 2 mm.

Nevertheless, a certain number of disadvantages are observed with it.Firstly, the assembling and the positioning of the graphite point on afilament whose diameter is less than 0.2 mm are very delicate and indeeduncertain. The assembly lacks mechanical stability, which causes thermaldrifting that is incompatible with applications in the field ofmicroelectronics. On the other hand, heating of the emissive area ispoorly controlled. The greater the thermal power used by the source, themore the thermal drift due to radiation to the environment is increased.This consequently, can modify the mechanical centring of the emissivepoint with respect to a target or with respect to an extractionelectrode. This centring influences the direction of the emission of theions and is inaccessible during operation. Its variation leads thereforeto a loss of precision; in the context of use in the field ofmicroelectronics, the resolution of the etched structures obtained withsuch a source is affected to a considerable extent. Another problem isthe limited life of the source. In effect, the reserve resolution of theetched structures obtained with such a source is affected to aconsiderable extent. Another problem is the limited life of the source.In effect, the reserve of liquid supply metal is small and cannot beused in its entirety.

Document WO 86/06210 describes another type of ion source, that includesa point and a heating element in the form of a tape. A hole in theheating tape allows a molten liquid to flow in the direction of thepoint.

DESCRIPTION OF THE INVENTION

This invention seeks to resolve the problems mentioned above.

The object of the invention is a liquid metal ion source including acylindrical rod made of a conductive and refractory material, extendedby a point made of refractory material, intended to be covered by aliquid supply metal, characterised in that the assembly made up by thecylindrical rod and the point passes through a reservoir made of aconductive material, the area where the rod is inserted into thereservoir ensuring an electrical contact between the rod and thereservoir, and in that the reservoir is in contact with a conductivefilament, the cylindrical rod, the reservoir and the conductive filamentbeing thereby connected in series from the electrical point of view.

Such a construction for a liquid metal ion source allows one to limitthe input of energy necessary for an optimal operation of the source. Ineffect, there is only production of heat in a very localised area,limited to the part of the cylindrical rod next to the point, to thereservoir and to the tungsten filament. The most resistive element inthe circuit is thus the cylindrical part of the rod, which, when acurrent of 5 amperes passes through it reaches a temperature of 700° C.adjacent its end by the Joule effect.

According to one particular embodiment of the invention, the rod and thepoint can be formed as one and the same piece.

For example, the rod and the point can be made of graphite.

According to one further particular embodiment, the graphite point iscovered with a film of titanium.

According to one variant, the point is covered with a metal priming coatof the same kind as the liquid metal it is intended to use with thesource.

The priming coat can not only cover the point but also a part of thereservoir.

In the case where the liquid metal intended to be used to cover thegraphite point is aluminium, a surface treatment of the point allowsimprovement to the wettability of the point by the liquid aluminium. Twosurface treatments are described below, the second has the advantage ofallowing one to obtain a very homogeneous film of aluminium over thewhole of the point. In effect, the surface treatment of the graphitepoint by deposition of a film of titanium does not allow one to achievehomogeneous wetting of the point with a film of aluminium: in fact, aformation of small islands of aluminium is obtained on the surface ofthe graphite point and as a result the function of supplying aluminiumto the point is very much disrupted. The current emitted is thenunstable and very difficult to keep constant over long periods.

Contrary to this, the second treatment on the one hand improves thefunction of supplying the metal to be ionised to the apex of the pointand, on the other hand, allows the stored quantity of supply metal to beincreased. The ion current then obtained from the production ofaluminium ions is then accordingly more stable over time.

According to another particular embodiment of the invention, thecylindrical rod and the point are mechanically adjusted with tightclearances to the inside of the area provided for their passage in thereservoir.

This adjustment gives the following advantage. During operation of thesource, it is possible that hot spots, other than those situatedadjacent the point, appear on the cylindrical rod, on the other side ofthe reservoir in relation to the point. If a certain clearance existsbetween the cylindrical rod and the reservoir, the liquid metal goesinto it and rises towards these hot spots, from whence it can evaporate,which reduces the life of the source. The adjustment, without anyclearance at all of the rod with the inside of the reservoir allows thisdisadvantage to be remedied.

Other complementary aspects of the invention appear in the dependentclaims.

DESCRIPTION OF THE FIGURES

The characteristics and advantages of the invention will become moreapparent in the light of the description which will follow. Thisdescription is supported by examples given for explanatory purposes andwhich are non-limitative, and makes reference to appended drawings inwhich:

FIG. 1, already described, shows a liquid metal ion source according tothe prior art,

FIG. 2 shows a liquid metal ion source according to this invention,

FIGS. 3a and 3b show two examples of a reservoir used in a liquid metalion source according to this invention,

FIG. 4 shows an embodiment of a rod for a source according to theinvention,

FIG. 5 shows the treatment device used to prepare a liquid metal ionsource according to this invention,

FIGS. 6 and 7 give examples of results obtained with an ion sourceaccording to this invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

FIG. 2 illustrates one particular embodiment of a liquid metal ionsource according to this invention.

This source is made up of a conducting rod comprising a cylindrical part10 and a point, the latter being itself made up of a conical part or end12 and a cylindrical part 13. The cylindrical part 10 is made of aconductive and refractory material. Generally, graphite is suitablewithin the context of an application with a light metal ion source, suchas aluminium. But this does not exclude the use of other materials, suchas, for example tungsten. The rod, in its cylindrical part 10, can have,in the case of graphite, a diameter of the order of a few tenths of amillimetre, for example 0.5 mm, and a length of between 5 and 20 mm, forexample 15 mm. In fact, in various different embodiment examples, asimple HB pencil lead has been used, which has given completesatisfaction.

The extremity 12 of the point is shaped into a cone, the half angle atthe apex having a value between 48 and 50°, for example 49°, bymechanical polishing in two stages. The point is set in rotation,inclined at about 49.5°, and it is brought into contact with a planewhich will machine the conical part. In a first stage, the cone isformed on a surface of medium roughness of about 30 μm. In a secondstage, finishing is carried out to a surface of low roughness, forexample a few microns.

In this way, at the apex of the point, a radius of curvature is obtainedof the order of about ten microns, in a reproducible manner.

The point, that is to say the whole item made up of the cylindrical part13 and the conical end 12 has a total length of between 5 and 10 mm, thecylindrical part 13 having a length of a few mm (for example 3 mm).

So as to avoid, during operation of the source, a rupture of the film ofliquid metal on the edge at the base of the cone, where the cylindricalpart begins, it can be of interest to chamfer the area 11 where thecylindrical part 13 joins the conical end 12.

If the point is made of the same material as the cylindrical part 10 ofthe rod, the whole can be one and the same piece.

The rod is introduced into a reservoir 14, two examples of which areillustrated in more detail in FIGS. 3a and 3b. In each of these figures,the reservoir designated by reference numbers 14-1 and 14-2 has a shapesubstantially having rotational symmetry about an axis passing through10. In the two examples, a cylindrical opening 19, on the inside of thereservoir, allows passage and also the holding of the rod in a fixedposition and contributes to the mechanical stability of the assembly. Inthe example in FIG. 3b, the reservoir 14-2 has a part cutaway in theform of a chamfer. This cutaway has the purpose of allowing the capacityof the reservoir for liquid metal to be increased. An internal void 17allows the volume of material used for the reservoir to be reduced.

In all cases, the reservoir is made of a conductive and refractorymaterial. If the rod 10 is graphite, it will be enough to select, forexample, graphite as the material for the reservoir.

One particularly advantageous form of graphite is vitreous carbon. Asopposed to polycrystalline graphite, which has micropores andmicrofissures, vitreous carbon has an impermeable structure of closedpores.

Over the long term, ageing of the source has been observed. It showsitself by the appearance of graphite powder on the surface of the liquidsupply metal, in particular in the case of aluminium.

The grains of this powder are, it would appear, lifted away by thechemical action of the molten aluminium, which penetrates into themicropores and the microfissures of the polycrystalline graphite, evenwhen specifically developed for refractory applications.

Tests carried out with a point and a reservoir made of vitreous carbonhave made it possible to observe:

the absence of carbon particles on the surface of the liquid metal film(aluminium, notably), which leaves one to think that the longevity ofthe source will be increased,

a clearly greater resistivity of the point-reservoir assembly, whichallows one to decrease the value of the heating current by a factor of2. This current is therefore reduced to about 2 amperes.

Furthermore, the possibility must be mentioned of electrochemicallyshaping the end of the vitreous carbon point in order to match thethreshold ionic emission value to a specific apparatus.

It is possible to work only with the point or the reservoir made ofvitreous carbon.

If only the reservoir is vitreous carbon, it is observed that there isno longer any corrosion of the reservoir. The resistivity is not so goodas in the preceding case but better than if the reservoir is made of"normal" graphite.

Similarly, if only the point is vitreous carbon, its life will beprolonged and the resistivity will be improved. Furthermore, the pointalone can still be shaped electrochemically.

The rod 10 being used as an electrical conductor, there is, inprinciple, production of heat in a very localised area, limited to theconical part 12, to the cylindrical part 13 and to the reservoir 14.However, occasionally the formation of "hot spots" cannot be avoided onthe cylindrical part 10, and close to the base of the reservoir 14, forexample, at a point such as point A shown in FIG. 2. At such a hot spot,the temperature can reach a value greater than that of the temperatureat the end of the rod, adjacent the point 12, due to the fact thatadjacent this point, the presence of liquid metal contributes to thedissipation of the heat. The liquid metal can have a tendency to diffusethermally, along the rod, in the direction of the hot spots which hasthe effect of gradually emptying the reservoir and hence of reducing theendurance of the source. For these reasons, it can be of interest toadjust mechanically the cylindrical part 10 of the rod with tightclearances, without any play, to the inside of the reservoir 14.Consequently, the liquid metal cannot escape along the rod. Furthermore,this reinforces the mechanical strength of the assembly. As an example,a reservoir has been made with a diameter of about 5 mm, for a height ofabout 2 mm. The cylinder 19 is machined to the nominal diameter of therod 10 and this is then forcibly introduced by hand. This is sufficientto ensure the required tight fit.

As illustrated in FIG. 2, the reservoir 14 is connected electrically toa heating circuit. This circuit can be made up, for example, of atungsten filament 16 wound around the base of the reservoir 14, on theopposite side to the point 12. The ends of this filament are themselvesconnected to conductive elements 24, 26, such as, for example, tantalumplates. According to a variant, not shown in the Figures, the filament16 is not in contact with the external surface of the reservoir 14, butit is introduced into a throat formed in the surface of this reservoir.This allows one to limit the contact between the heating filament 16 andpossible drops of liquid metal that could have diffused along theexternal surface of the reservoir 14. In effect, certain liquid metals,and notably aluminium, are extremely corrosive with respect to metals.

The rod is held at its base by a clamp made up of two jaws 28, 30 whichhave the purpose, on the one hand of ensuring that the rod is heldmechanically with maximum rigidity without making it brittle and, on theother hand ensuring a reliable electrical contact with the material ofthis rod, electrical contacts that allow the circulation of a current ofabout 6 A.

Made up in this way, the heating circuit includes the rod, with itscylindrical part 10 and its point, the reservoir 14 and the filament 16.From the electrical point of view, all of these elements are connectedin series and the assembly operates at a floating power supply voltageof the order of a few volts. The most resistive element of the circuitis the cylindrical part of the rod 10 which when a current of 5 A passesthrough it, reaches a temperature of 700° C. through the Joule effect.Furthermore, the fact that the rod is used as a heating element allowsthe power consumed to be limited to less than 10 Watts approximately.

The assembly rests on a baseplate 32, with 3 threaded rods 27-1, 27-2,27-3 passing through it. The central threaded rod 27-2 is extended bythe jaws 28, 30; the side rods 27-1 and 27-3 are extended by the fixingjaws 29-1 and 29-2 for the plates 26 and 24. All these elements (jaws,threaded rods) are part of the electrical circuit.

The structure which has just been described confers very good mechanicalstability on the assembly, and notably to the point 12. This allows theemissive area at the apex of the point to be stabilised and makes thesource compatible with use in technical fields where the precisionrequired is extremely high, for example in electrostatic optics.Furthermore, from the point of view of the space it occupies, the sourcedescribed is totally compatible with the machines and systems thatalready exist to which liquid metal ion sources are fitted.

Another embodiment of the rod is shown in FIG. 4. In this Figure, therod passes through the reservoir 15. The latter is similar to thatdescribed above in connection with FIG. 3b, except for the cylindricalopening 33 which has a diameter larger in its upper part than in itslower part, thereby defining a shoulder 34.

The rod is still made up of a cylindrical part 35. It is extended by apoint which is itself made up of a conical part 37, a cylindrical part39 and an edge 41. The rod 35 is of a diameter substantially greaterthan that in the first embodiment of the rod. The cylindrical part 39and the extremity 37 of the point have approximately the same dimensionsas previously.

In this embodiment, the cylindrical part and the point are made of twodifferent materials.

The rod is made of a conductive and refractory material, for example,graphite. As in the first embodiment, it can be made from a pencil lead.It is introduced into part of the depth of the cylindrical opening 33,in such a way that it is in contact with the point.

The point is made of a refractory material such as boron nitride oralumina. It is introduced in such a way that the edge 41 rests on theshoulder 34 and it is in contact with the end of the cylindrical part35.

The point 37 is shaped with a half-angle at the apex of between 48 and50° (with a value for example of 49°) by mechanical polishing using anabrasive such as, for example, a diamond grinding wheel.

This second embodiment can also be used in combination with a reservoirof a shape similar to that described in connection with FIG. 3a, withthe condition that the cylindrical opening is adapted in a correspondingmanner (opening of the cylinder larger in its upper part than in itslower part). What has been said in the context of the first embodimentabout the mechanical adjustment with tight clearances also applies tothis second embodiment.

The operation of the source is the same; there is still electricalcontact between the rod 35 and the reservoir 15 and the current flowsfrom the rod to the reservoir and to the heating filament. This currentcauses heating of the point through the Joule effect.

Whatever the shape of the rod, and so as to improve the wettability ofthe point 12 and of the reservoir by the liquid supply metal, it is ofinterest to carry out a surface treatment of this point and of thereservoir. First, the surfaces to be treated may have been cleaned in abath of boiling trichlorethylene, then degassed by heating under vacuumat a temperature of about 1000° C.

A surface treatment has been described in the article by Bell et al.already mentioned above. This treatment consists of laying down anaqueous solution of titanium powder on the point. After drying, thepoint is brought, under vacuum, to a temperature of about 1700° C. tomelt the titanium. This treatment is compatible with the structure ofthe source according to this invention, such as described above.

A variant of this surface treatment consists of depositing the film oftitanium by spraying before bringing it, under vacuum, to a temperatureof about 1700° C. to melt it. In the case of the use of the source withliquid aluminium, it has been observed that, with this surfacetreatment, a clearly improved homogeneity of the aluminium film isobtained compared with that obtained in the case where the surface istreated by the process described above. This allows one to have a goodsupply to the apex of the point and hence a more stable ion current overtime.

A third method of treating the surfaces that can be used in the contextof this invention, consists of irradiating with a beam of ions the pointand the reservoir designed to receive the liquid supply metal. In thecase where the source is intended to be used with liquid aluminium, thebeam of ions is a beam of aluminium ions. The irradiation is carried outwithin an enclosure under vacuum. FIG. 5 shows schematically theimplementation of the irradiation process. The source whose extremitymust be irradiated is shown on the right of the Figure, in a verticalposition, held by a support 40, which can pivot about a vertical axis,and which can also be displaced in translation along the threeperpendicular directions of the space. So as to carry out the desiredirradiation of the extremity of the source 38, another source of ions 42must be made available. This source can either be a source identical tothat which is sought to be produced and to which a similar treatment hasalready been applied, or a source such as that described in the priorart, for example, in the article by Bell et al. already mentioned above.An extraction electrode 44, permits the acceleration of the ions formedby the source 42 in the form of a beam 48 which passes through a windowmade in the electrode 44. This window includes an extraction diaphragm47. In order to regulate the extraction current of the source 42, theemitting point is held at a variable high voltage, of about 10-12 kV.The assembly formed by the extraction electrode 44 and the source 42 canbe oriented in space according to three perpendicular directions. Ingeneral, the beam of ions 48 has a conical shape, such as that shown inFIG. 4, and it is along the central axis of this beam that the maximumcurrent is apportioned. Therefore, it is of interest to position thesource 42 in such a way that the part of the source 38 to be irradiatedis approximately on the central axis of the ion beam 48. The dose ofions received by the source 38 during the treatment corresponds roughlyto a surface dose of 10¹⁸ ions/cm², that is to say to an irradiationwith a current of 2 μA for 1 hour. The acceleration voltage applied tothe aluminium ions formed by the source 42 can vary between a fewkilovolts and 20 kilovolts; it can, for example, have a value of about12 kilovolts.

The surface treatment is mainly carried out in two stages:

1. an etching stage, during which the source 42 emits a current of a fewmicroamperes, between 5 and 10 μA, that is regulated by adjusting thehigh voltage at which the emitting point of the source 42 is held. Infact the upper limit value of this current can be chosen in such a waythat, in the first stage, there is essentially the emission of simpleAl⁺ ions, and practically no emission of Al⁺ _(n) aggregates. With anacceleration voltage between the emitting point and the extractionelectrode 44, of about ten kilovolts, the aluminium Al⁺ ions are goingto:

etch the surfaces of the source 38 which are exposed to their path (thepoint 52 and the end of the reservoir 54),

form a layer on the etched surface, which will be used as a priming coatfor the aggregates which will be deposited during the second stage,

2. in a second stage, one causes a greater current to be beamed to thesource, a current sufficient to form a beam of metal aggregates of theAl_(n) ⁺ type in the case of aluminium. In general, a current of valuegreater than 50 μA is sufficient to form these aggregates. Theseaggregates are going to be deposited onto the layer of metal ions laiddown in the first stage onto the treated surface and which are playingthe role of a priming coat. This layer of metal aggregates itself formsa priming coat for the liquid metal which must subsequently be depositedon the end of the source.

The duration of each of the two stages described above depends on thecurrent used in each stage. With a current of a few microamperes, thefirst stage has a duration of about 20 minutes; for a current of about50 μA, the second stage takes a period of about 40 minutes.

It is possible to add to these two stages a stage of implanting ions andmetal aggregates already deposited on the surface treated in the firsttwo stages. During this implantation stage, the source 42 emits acurrent of a few microamperes, in such a way that a beam, principallymade up of simple ions with few aggregates, is emitted. These ions areaccelerated by a maximum voltage (of the order of 20 kilovolts) in sucha way that the aggregates deposited during the second stage are "driveninto" the superficial part of the area which has been treated during thepreceding stages.

In order to limit the surface treatment to the point 52 and the frontpart of the reservoir 54 (the part with reference numbers 20 and 31 inFIGS. 2 and 4 and which is outlined in these same Figures by a dottedline), it is possible to interpose a protective foil 50, such as, forexample, aluminium foil, between the beam 48 and the parts of the source38 that one does not want to irradiate. This can be important, in thecase where the liquid metal with which the source is intended to be usedcan have corrosive effects on the metal parts of the source. This isnotably the case with aluminium which, in the liquid state, can easilycorrode the parts of the heating system for the reservoir and the pointwhich are external, notably the tungsten filament (see FIG. 2). If, as aconsequence, a priming coat made up of metal aggregates is deposited onthese parts during the irradiation, the liquid metal will have atendency, during use of the source prepared in this way, to adhereequally well to these parts, which will bring about rapid corrosion ofthe metallic elements adjacent to these parts. It is for this reason,notably in the case of aluminium, that the irradiation of the source islimited to the point and the front part of the reservoir and to thewalls of the open compartment 18 in the reservoir (see FIG. 3b). Afterhaving been subjected to this treatment, the source is ready for use. Itis immersed, for example, in a bath of liquid aluminium which will wetthe irradiated parts and, by capillarity, cover them with a perfectlyhomogenous and uniform thin film. This very good homogeneity, on the onehand encourages the supply of ionised metal to the apex of the pointand, on the other hand, allows one to maximise the quantity of storedsupply metal.

This third treatment, applicable to a source having a structureaccording to the invention can also be applicable to a source having thestructure described in the article by Bell et al. and illustrated inFIG. 1. In this case, it is sufficient to subject the graphite point 2to an irradiation with, for example, a beam of aluminium ions (Al⁺ thenAl_(n) ⁺). The wettability of the graphite is improved compared with thetreatment proposed by Bell et al. in the article mentioned previously,since the latter leads to the formation of small islands of aluminium onthe surface of the graphite point.

The invention has been described in the context of the creation of asource of aluminium ions. The choice of this element is not limitativeand the same structure and the same surface treatment can be used forany source of ions of another kind, for example, for a boron source. Thesurface treatment will then consist of irradiating the source with abeam of boron ions, firstly B⁺ ions and then B_(n) ⁺ aggregates. Boronis similarly a corrosive element in the liquid state like aluminium andit is therefore preferable to limit the surface treatment to thegraphite point 12 and the "front" part of the reservoir 14.

The structure of the source according to this invention can equally wellbe used for the production of ions from other elements, notably elementsnoncorrosive in the liquid state.

After preparation and once the source is wetted by the liquid metal, theproduction of ions from it is carried out using an extraction electrode,mounted in front of the point, in the same way as the electrode 44 ismounted in front of source 42 in the assembly in FIG. 5.

The beam obtained can be more or less rich in metal aggregates ofvariable size. In fact this choice depends on the voltage applied to thepoint. For this reason, the source assembly is held at a high voltage ofabout 11 kV, the supplementary high voltage supply being connected tothe base of the rod 10 (using jaws 28, 30 in the representation shown inFIG. 2). The modulation of the high voltage leads to a modulation of thecurrent emitted by the point, this current, in its turn, modulating thedistribution of the size of aggregates emitted. As for the potentialdifference between the point and the extraction electrode, thismodulates the kinetic energy of the ions or aggregates emitted.

The main applications of the source according to this invention are:

on the one hand the manufacturing of focused ion beam machines

on the other hand, the use of such machines in the field ofmicroelectronics and for the preparation of samples for viewing bytransmission microscopy (TEM).

The principle consists then of using the interaction between a beam ofvery energetic ions, focused on a spot of less than 0.1 micron, and asample.

The incident ions are going to pulverise the surface of the samplelocally on the spot corresponding to the impact area.

The erosion process is controlled along X, Y axes parallel to thesurface of the sample by sweeping this surface with the beam, and thedepth of this etching by displacing the machine along a z axisperpendicular to the surface. Structures having sizes of the order of 70to 80 nanometers can be developed with an apparatus incorporating asource according to this invention.

Examples of results obtained with an aluminium ion source designedaccording to this invention are given in FIGS. 6 and 7. The source usedincorporated a surface treatment with a priming coat of aluminium asdescribed above.

FIG. 6 is a photograph of a copper grid taken using an electronmicroscope in which the electron gun of the microscope has been replacedby the source of aluminium ions. The acceleration voltage of the ionswas 12.5 keV, and the emission current was 16 μA. The wires 60, 62, 64of the grid have a thickness of about 25 μm. To create such a shot, thegrid had to be irradiated with a very stable beam of Al⁺ ions for about1 minute 30 seconds. Hence, this photograph shows the very goodstability, over time, of the current and of the beam from the sourceaccording to the invention. FIG. 7 is a photograph of an etching carriedout on GaAs by a beam of Al⁺ ions of 20 kev energy (current i≈11 μA). Inthis photo, 1 cm represents 100 nm.

We claim:
 1. A liquid metal ion source including a reservoir made of anelectrically conductive material for holding a supply of the sourcemetal in liquid state, a cylindrical rod, made of an electricallyconductive refractory material, having a conical pointed end, a portionof the rod extending through an aperture in the reservoir in contactwith the supply of metal in liquid state, and an electrically conductivefilament in electrical contact with the reservoir, wherein thecylindrical rod, the reservoir and the conductive filament areelectrically connected in series so as to constitute a heating circuitfor said liquid metal when a current passes through it, the mostresistive element of said heating circuit being the cylindrical rod, andthe portion of the rod extending through the aperture in the reservoirhas a tight clearance with the walls defining the aperture in thereservoir.
 2. A liquid metal ion source according to claim 1, whereinthe reservoir is formed of an electrically conductive refractorymaterial.
 3. A process for preparing a sample using a liquid metal ionsource comprising a reservoir made of an electrically conductivematerial for holding a supply of the source metal in the liquid state, acylindrical rod, made of an electrically conductive refractory material,having a conical pointed end, a portion of the rod extending through anaperture in the reservoir in contact with the supply of metal in liquidstate, and an electrically conductive filament in electrical contactwith the reservoir, wherein the cylindrical rod, the reservoir and theconductive filament are electrically connected in series so as toconstitute a heating circuit for said liquid metal when a current passesthrough it, the most resistive element of said heating circuit being thecylindrical rod, and a beam of metal ion is produced using said sourceand directed onto the sample.
 4. A process according to claim 3, whereinthe rod and the point are adjusted mechanically with tight clearances tothe inside of the area provided for their passage in the reservoir.
 5. Aliquid metal ion source comprising:a reservoir made of an electricallyconductive material for holding a supply of the source metal in theliquid state, a cylindrical rod, made of an electrically conductiverefractory material, having a conical pointed end, a portion of the rodextending through an aperture in the reservoir in contact with thesupply of metal in liquid state, and an electrically conductive filamentin electrical contact with the reservoir, wherein the cylindrical rod,the reservoir and the conductive filament are electrically conductivefilament in electrical contact with the reservoir, wherein thecylindrical rod, the reservoir and the conductive filament areelectrically connected in series so as to constitute a heating circuitfor said liquid metal when a current passes through it, the mostresistive element of said heating circuit being the cylindrical rod. 6.A liquid metal ion source according to claim 5, wherein the conductivematerial from which the reservoir is formed comprises a refractorymaterial.
 7. A liquid metal ion source according to claim 5, wherein therod and the point are formed of one and the same piece.
 8. A liquidmetal ion source according to claim 7, wherein the rod and the point areformed of graphite.
 9. A liquid metal ion source according to claim 8,wherein the point is covered with a film of titanium.
 10. A liquid metalion source according to claim 5, wherein the rod and the point areformed of different materials.
 11. A liquid metal ion source accordingto claim 10, wherein the rod is formed of graphite.
 12. A liquid metalion source according to claim 10, wherein the point is formed of aluminaor boron nitride.
 13. A liquid metal ion source according to claim 5,wherein the point is covered with a metallic priming coat, which metalis the same as the liquid supply metal.
 14. A liquid metal ion sourceaccording to claim 13, wherein the priming coat covers the point, and apart of the reservoir.
 15. A liquid metal ion source according to claim13, wherein the priming coat is formed by ionic bombardment.
 16. Aliquid metal ion source according to claim 15, wherein the ionicbombardment is carried out in two stages:an etching stage, during whicha beam of ions containing essentially no aggregates is directed onto theparts to be irradiated from an ion source, and a second stage, duringwhich a beam of ions comprising essentially metallic aggregates isdirected onto the parts to be irradiated from the ion source.
 17. Aliquid metal ion source according to claim 16, and further comprising animplantation stage, the parts of the source to be irradiated beingbombarded by a beam comprising mainly ions accelerated under a highvoltage, and few aggregates.
 18. A liquid metal ion source according toclaim 5, wherein the point is formed of vitreous carbon.
 19. A liquidmetal ion source according to claim 5, wherein the reservoir is formedof vitreous carbon.
 20. A liquid metal ion source according to claim 5,wherein the reservoir has a cutaway hollow.
 21. A liquid metal ionsource according to claim 5, wherein the heating filament is introducedinto a throat at the periphery of the reservoir.